backlight: oops in __mutex_lock_slowpath during head /sys/class/graphics/fb0/* in 2.6.18

[Olaf Hering <olaf@aepfle.de>]

The bl_curve code has some room for improvement.

...
<6>Monitor sense value = 0x60b
<6>using video mode 13 and color mode 0.
<4>Console: switching to colour frame buffer device 104x39
<6>fb0: valkyrie frame buffer device
...

inst-sys:~ # cat /proc/fb
0 valkyrie
inst-sys:~ # l /sys/class/graphics/fb0/
total 0
drwxr-xr-x 2 root root    0 Sep 21 11:54 ./
drwxr-xr-x 4 root root    0 Sep 21 11:53 ../
-rw-r--r-- 1 root root 4096 Sep 21 12:15 bits_per_pixel
-rw-r--r-- 1 root root 4096 Sep 21 12:15 bl_curve
-rw-r--r-- 1 root root 4096 Sep 21 12:15 blank
-rw-r--r-- 1 root root 4096 Sep 21 12:15 console
-rw-r--r-- 1 root root 4096 Sep 21 12:15 cursor
-r--r--r-- 1 root root 4096 Sep 21 12:15 dev
-rw-r--r-- 1 root root 4096 Sep 21 12:15 mode
-rw-r--r-- 1 root root 4096 Sep 21 12:15 modes
-r--r--r-- 1 root root 4096 Sep 21 12:15 name
-rw-r--r-- 1 root root 4096 Sep 21 12:15 pan
-rw-r--r-- 1 root root 4096 Sep 21 12:15 rotate
-rw-r--r-- 1 root root 4096 Sep 21 12:15 state
-r--r--r-- 1 root root 4096 Sep 21 12:15 stride
lrwxrwxrwx 1 root root    0 Sep 21 12:15 subsystem -> ../../../class/graphics/
--w------- 1 root root 4096 Sep 21 12:15 uevent
-rw-r--r-- 1 root root 4096 Sep 21 12:15 virtual_size
inst-sys:~ # head /sys/class/graphics/fb0/*
==> /sys/class/graphics/fb0/bits_per_pixel <==
8

==> /sys/class/graphics/fb0/bl_curve <==
Segmentation fault
inst-sys:~ # dmesg
valkyriefb: vmode 13 does not support cmode 1.
Unable to handle kernel paging request for data at address 0x00000000
Faulting instruction address: 0xc02c7bd8
Oops: Kernel access of bad area, sig: 11 [#1]

Modules linked in: tulip mesh cpufreq_ondemand loop nfs nfs_acl lockd sunrpc sg st sd_mod sr_mod scsi_mod ide_cd cdrom
NIP: C02C7BD8 LR: C0170EBC CTR: C0170E90
REGS: c4209db0 TRAP: 0300   Not tainted  (2.6.18-rc7-git1-2-default)
MSR: 00009032 <EE,ME,IR,DR>  CR: 28042488  XER: 00000000
DAR: 00000000, DSISR: 22000000
TASK = c7fc8190[2853] 'head' THREAD: c4208000
GPR00: C4209E6C C4209E60 C7FC8190 C0B77614 C41D5000 C41D5000 00000000 00008000
GPR08: 00000000 C0B77618 00000000 00000000 00000000 100200C8 10020000 00000001
GPR16: 10006D08 7FDF9B3C 100A0000 10080000 00000003 7FDF9CAB C4209F20 C2FCDC78
GPR24: C038BC14 C01EF568 7FDF7858 C41D5000 C0B77614 C0B77400 C7FC8190 C0B77614
NIP [C02C7BD8] __mutex_lock_slowpath+0x2c/0xa4
LR [C0170EBC] show_bl_curve+0x2c/0xa0
Call Trace:  
[C4209E60] [C02C7A54] mutex_lock+0x18/0x5c (unreliable)
[C4209E80] [C0170EBC] show_bl_curve+0x2c/0xa0
[C4209EB0] [C01EF594] class_device_attr_show+0x2c/0x44
[C4209EC0] [C00CC3BC] sysfs_read_file+0xb8/0x204
[C4209EF0] [C0083FE4] vfs_read+0xec/0x1c8
[C4209F10] [C0084444] sys_read+0x4c/0x8c
[C4209F40] [C00125F8] ret_from_syscall+0x0/0x40
--- Exception: c01 at 0xff589f8
    LR = 0x100045e0
Instruction dump:
4e800020 9421ffe0 7c0802a6 39230004 bfc10018 7c7f1b78 7c5e1378 90010024
3801000c 9121000c 81690004 90090004 <900b0000> 91610010 90410014 3800ffff